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Fig. 1
Comparison of the experimentally determined geometries of the hydrogen-bonded
complex H
3
N
ClF (both drawn to scale)
with a non-bonding electron-pair (n-pair) model of NH
3
. Here, and in other figures, the
n-pair electron distribution is drawn in the exaggerated style favoured by chemists. The
key to the colour coding of atoms used in this and similar figures is also displayed
···
HCl and its halogen-bonded analogue H
3
N
···
3.1.2
B Carries Two Equivalent n-Pairs
3.1.2.1
Linear Halogen Bonds and Hydrogen Bonds
The prototype Lewis bases in this category are H
2
OandH
2
S. The complex
H
2
O
HF is sufficiently strongly bound to have been investigated in an equi-
librium mixture of H
2
OandHFheldat200 K in the cell of a conventional
Stark-modulation microwave spectrometer [111, 112]. This allowed vibra-
tional satellites associated with low-frequency, intermolecular stretching and
bending modes to be observed and analysed and vibrational wavenumbers
for these modes to be determined. It was not only possible to conclude that
in the zero-point state this complex is effectively planar but also to determine
the potential energy (PE) as a function of the out-of-plane, low-frequency,
hydrogen-bond bending co-ordinate. The mode in question inverts the con-
figuration at the oxygen atom and is shown schematically in Fig. 2. The
O
···
H - F nuclei were assumed to remain collinear during this motion. The
energy levels associated with the motion were calculated by using the ex-
···
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